This disclosure relates generally to resistors that may be used in electrical circuits, and more specifically to resistors with a range of discrete resistance states. Also, this disclosure pertains to electrically programmable analog variable resistors and circuits using devices exhibiting electrically programmable, analog resistances.
Typically, in an integrated circuit, resistors are fabricated by doping a material, such as silicon, with another material, such as phosphorus or boron, to a level that achieves a suitable resistance value in the material. The resistance of a device fabricated from this material is generally static in the final integrated circuit. Another way of achieving a suitable resistance value in an integrated circuit is by biasing a transistor, such as a MOSFET, at certain conditions specific to the transistor. For example, a MOSFET may be operated in the triode region to achieve a specific resistance. Unlike a passive resistor, actively biasing a transistor requires constant power. Neither technique makes it possible to create a range of available resistance states that retain their state in the absence of an applied electrical signal, during normal operation of an integrated circuit.
Chalcogenide glasses containing an excess of metal, have been shown to exhibit negative differential resistance (NDR). In this context, NDR is the same as differential negative resistance (DNR)—referred to in the prior art. U.S. Pat. Nos. 7,329,558, 7,050,327, and 7,015,494 describe devices displaying NDR/DNR, using these devices in binary electronic memory, and as an analog memory via the current value read at the NDR peak. In the prior art, the NDR device is formed by addition of an excess of metal ions in a chalcogenide glass by either heating a chalcogenide material layered with a metal layer or by application of a fast electrical pulse with pulse width and amplitude specific to the chalcogenide material type. The peak current is programmed by application of a pulse of duration and amplitude that can cause the peak current to be either reduced or increased. The device current is read at or near the voltage corresponding to the peak current value.
NDR devices can be fabricated with standard complementary metal-oxide semiconductor (CMOS) processes at sizes consistent with the state of the art feature sizes, thus integrating well with existing and future integrated circuit technologies. Conventional chalcogenide devices may be comprised of GexSe1-x, wherein 0≦x≦0.9. Some chalcogenide devices contain copper and/or silver and/or mixtures thereof. For example, chalcogenide devices may comprise a combination of (GexSe1-x)yCu1-y and (GexSe1-x)yAg1-y, wherein 0≦x≦0.9 and wherein 0.1≦y≦0.9. Some chalcogenide devices are of a single layer of chalcogenide material containing an excess of metal which causes the NDR response.
Chalcogenide materials have also been explored for creating phase change devices. Phase change devices take advantage of a property of some chalcogenide materials that allows some or all of the material to be physically changed between crystalline or amorphous states. By changing the physical structure of a chalcogenide, one or more electrically measureable parameters of the material, such as the resistance, may change.
Ion-conduction through chalcogenide material is also known. A voltage pulse may be applied to an ion conduction device to move metal into a chalcogenide material, forming a continuous and conductive path of metal through the chalcogenide material (i.e., forming a relatively low resistance state). The resistance of the conductive path can be altered repeatedly.